The present invention relates to rack and pinion steering devices for use in motor vehicles and the like.
FIG. 10 shows a rack and pinion steering device already known for use in motor vehicles.
With reference to FIG. 10, a pinion shaft 5 is rotatably supported by a housing 1 with bearings 2, 3 and an oil seal 4 provided therein around the shaft. Meshing with a pinion 6 formed at an intermediate portion of the pinion shaft 5 is a rack 8 formed on the rear side of a rack bar 7 extending through the housing 1 transversely thereof. The housing 1 is integral with a hollow cylindrical portion 9 projecting forward from the front side thereof. A cap 10 is screwed in the front end of the cylindrical portion 9 and fixed thereto with a lock nut 11. A rack guide 12 is fitted in between the cap 10 and the rack bar 7 in the cylindrical portion 9 so as to be slightly movable forward or rearward. The rack guide 12 is formed in its rear end with a guide groove 13 having the cylindrical front face of the rack bar 7 fitted therein to guide the bar 7. The guide groove 13 is defined by upper and lower rearward projections 14. The cap 10 and the rack guide 12 are formed in their opposed faces with circular spring accommodating cavities 15, 16, respectively, having a coiled compression spring 17 disposed therein. The rack guide 12 is pressed against the rack bar 7 by the spring 17 to guide the rack bar and preload the rack 8 relative to the pinion 6. A small clearance is provided between the cap 10 and the rack guide 12.
Although the rack guide was prepared from sintered alloy or the like in the past, those made of synthetic resin have been placed into wide use in recent years since they permit smooth movement of the rack bar, and are lightweight, diminished in impact noise and inexpensive. When the rack guide is prepared from synthetic resin, injection molding is generally resorted to.
While the rack guide must fulfill the requirements as to both strength and precision, the use of synthetic resin for the rack guide involves the following problems.
Insofar as the strength requirement is concerned, the rack guide will be satisfactory when made in the form of a block having no clearance, whereas the product then deforms greatly owing to the shrinkage involved in injection molding, hence a problem in respect of precision. Accordingly, the rack guide is prepared in the form of a hollow cylinder, but a problem then arises with respect to strength. More specifically stated with reference to FIG. 10, the rack guide 12 collides with the cap 10 secured to the housing cylindrical portion 9 when subjected to a force from the rack bar 7. At this time, the cylindrical portion only of the rack guide 12 is subjected to a force upon striking against the cap 10. The rack guide is therefore likely to break, failing to withstand the great force.
Furthermore, it is difficult to prepare the rack guide from synthetic resin by a single cycle of injection molding in view of its shape. Even if this is possible, another problem is encountered in that the product undergoes post-process changes (thermal expansion, thermal contraction and hygroscopic swelling) which involve directionality.
More specifically, in the case where the rack guide 12 of FIG. 10 is injection-molded, the molding tends to shrink during cooling at the rear end portion where the guide groove 13 is formed, and becomes tapered toward the rear end. If the rear end portion shrinks uniformly in its entirety in this case without any directionality in the degree of shrinkage of this portion, it is possible to make the entire rack guide 12 uniform in outside diameter by dimensionally adjusting the molding die. However, it is impossible to give a uniform outside diameter to the entire rack guide 12 by adjusting the dimensions of the die since the degree of shrinkage is actually greater in the direction in which the projections 14 approach each other. Additionally, the molding synthetic resin flows in the direction in which the guide groove 13 extends, with the result that the filler used, such as glass fiber, becomes oriented in the horizontal direction (i.e., the direction perpendicular to the plane of FIG. 10) in the portions forming the projections 14. Consequently, the filler fails to greatly reinforce the projections 14 by acting against the fall of these portions.